The influence of substrate on siderophore production by fish spoilage bacteria

Siderophore production of fish spoilage bacteria (5 isolates of Shewanella putrefaciens and 5 of Pseudomonas sp.) was determined in fish extract, Tris-succinate medium, a liquid medium of the Chrome-Azurol-S (CAS) agar and in M9 medium supplemented with glucose and casamino acids (M9GC). One Pseudomonas isolate produced siderophores in all four media whereas the other isolates varied. One isolate was negative in all media except for the M9GC that supported siderophore production for all five Pseudomonas isolates. Neither the hydroxamate nor catechol reaction was found for any of the Pseudomonas isolates. S. putrefaciens produced siderophores of the hydroxamate type in fish extract and to a lesser extend in the M9GC medium. Growth was supported by the other media. S. putrefaciens grew weakly on the Chrome-azurol-S (CAS agar as this medium did not support siderophore-production. However, the isolates formed orange halos on a fish extract agar to which CAS was added. Outer membrane proteins (OMPs) were analyzed by SDS-PAGE and, for each siderophore positive isolate, the same iron repressed OMPs were expressed in the different media supporting siderophore production. One S. putrefaciens isolate expressed an assumed receptor protein (69-72 kDa) in Tris-succinate medium in which a positive siderophore reaction as measured by the GAS-assay was not detected.